Structure of flash memory

ABSTRACT

A structure of a flash memory having a tunnel oxide layer with high reliability, low defect and interface trap, manufactured by using a process of semi-atmospheric pressure chemical vapor deposition (SPACVD) and tetra-ethyl-ortho-silicate (TEOS) reactant, wherein the SAPCVD process is performed accompanied with a reaction temperature between about 600° C. and about 750° C. and a reaction pressure between about 340 Torr and about 500 Torr to react TEOS and oxygen.

FIELD OF THE INVENTION

[0001] The present invention relates to a structure of a flash memory having interpoly dielectric layer for improving reliability of flash memory devices. More particularly, it is related to a structure of a flash memory manufactured by using a process of semi-atmospheric pressure chemical vapor deposition (SAPCVD) and tetra-ethyl-ortho-silicate (TEOS) reactants.

BACKGROUND OF THE INVENTION

[0002] Flash memory is a semi-conductor technique developed according to RAM product of computer. Flash memory is a solid-state storage system, which consumes less-power to change inner data quickly by using the efficient block way, and flash memory retains data without any additional power.

[0003] Flash memory and other solid-state memories, for example, read only memory (ROM), static/dynamic random access memory (SRAM/DRAM), and electrically erasable programmable read only memory (EEPROM) are applied widely.

[0004] Among these solid-state memories, the flash memory is the best storage system with high quality since the flash memory has characteristics of non-volatile, rewritable, high density and stability.

[0005]FIG. 1 is a cross-section view of a conventional flash memory device. The structure of the conventional flash memory device includes a substrate 10, and a source region 12, a drain region 14, and a channel region 16 located in the substrate 10, and a stacked gate structure 18 located on the substrate 10. Herein the stacked gate structure 18 further includes a tunnel oxide layer 20, a floating gate 22, an interpoly dielectric layer 24, and a control gate 26. The floating gate 22 and the control gate 26 are usually composed of polysilicon, and the interpoly dielectric layer 24 is composed of multi-insulated layers, i.e. oxide/nitride/oxide (ONO) structure. The interpoly dielectric layer 24 includes a bottom oxide layer 28, a nitride layer 30, and a top oxide layer 32. When current flows through the channel region 16 of the flash memory device to electrically connect the source region 12 and the drain region 14, meanwhile electric field is applied to the stacked gate structure 18.

[0006] To be the insulation structure between the floating gate 22 and the control gate 26, the interpoly dielectric layer 24 has to be high reliability. For example, if the top oxide layer 32 is too thick, the needful conductive voltage may be increased. Otherwise, if the top oxide layer 32 is too thin, in the flash memory the current leaks out easily so that the memory ability and storage time of charges also decreased. Accordingly, it is important to control the thickness of the top oxide layer. In addition, if the nitride layer 30 is too thin, current leakage may be happened between floating gate 22 and control gate 26, and charge storage time is also shortened.

[0007] The conventional method of manufacturing the interpoly dielectric layer 24 ONO structure of flash memory device is to oxidize the nitride layer 30 directly by wet thermal oxidation. For example, under 950° C. vapor circumstance the process of oxidizing the nitride layer about 40 minutes to transform a portion of the nitride layer into the top oxide layer 32.

[0008] The other conventional method of manufacturing the interpoly dielectric layer 24, ONO structure of flash memory device is to form an oxide layer on the nitride layer, for example, by low-pressure vapor deposition (LPCVD). The LPCVD process is performed under the circumstance, includes: lower temperature between about 600° C. and about 850° C. and higher pressure between about 400 mTorr and about 750 mTorr; injecting reactive gases (as SiH₄ and N₂O) and inert gas or N₂ to form SiO₂ layer; and performing a rapid thermal anneal (RTA) process to nitrify the oxide layer for about 40 seconds to 80 seconds under a temperature between about 700° C. and 950° C. in order to densify the oxide layer or reduce defects and charge trap formed on the top oxide layer of flash memory devices. Since the increased temperature decomposes N₂O to N₂ and reactive oxygen molecules, the oxygen molecules will diffuse to oxygen lattice's vacancies of LPCVD oxide layer resulting to density decreasing and current leakage.

SUMMARY OF THE INVENTION

[0009] In the above interpretation, the conventional method of wet thermal oxidation tends to react excess nitride, so that the nitride layer may be too thin resulting charge leakage. In the other hand, long reactive time and high reaction temperature also introduce defects and charge traps and decrease the reliability of tunnel oxide layer.

[0010] In the above interpretation, the conventional method of LPCVD process is more complex and consumes more gases by repeating decompression and gas exhausting to maintain low-pressure circumstance. Molecules in low-pressure move in the form of molecular flow, so that the collision frequency between molecules is very low and it is therefore very hard to produce the collisions needed to induce CVD process. Hence, the deposition rate of film is slow and takes long time. Moreover the turbulent flow also induces dust in reaction chamber so that the deposition quality is influenced. Furthermore, the SiH₄ reactant is reacted by homogeneous nucleation so that the step coverage ability of this conventional process is poor because of the surface pollution of the reactor by dust.

[0011] Therefore, one aspect of the present invention is to provide a semi-atmospheric pressure chemical vapor deposition process to improve the disadvantages of long thermal budget and uncontrolled thickness, which are caused from conventional wet thermal oxidation under high temperature. The present invention also improves the problems caused by repeating decompression, low collision frequency and turbulent flow.

[0012] Other aspect of the present invention is that using tetra-ethyl-ortho-silicate (TEOS) reactants to form the oxide layer to improve the disadvantage of dusted pollution caused from SiH₄.

[0013] According to the above aspects, the present invention provides a structure of a flash memory manufactured by the method combining SAPCVD process and TEOS reactants to decrease defect and interface trap, wherein SAPCVD includes the step of reacting TEOS reactants with oxygen under temperature between about 600° C. and 750° C. and pressure between about 340 Torr and 550 Torr. A structure of a flash memory comprises: a tunnel oxide layer located on a substrate; a floating gate located on the tunnel oxide layer; and an interpoly dielectric layer located on the floating gate, wherein the interpoly dielectric layer comprises an oxide layer, a nitride layer and a semi-atmospheric pressure chemical vapor deposition (SAPCVD) oxide layer, wherein the SAPCVD oxide layer is formed by a semi-atmospheric pressure chemical vapor deposition process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

[0015]FIG. 1 is a cross-section view of a conventional flash memory device; and

[0016]FIG. 2 is a cross-section view of a flash memory device according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] The present invention discloses a top oxide layer with a few defects and interface traps to improve reliability of interpoly dielectric layer of a flash memory device. The present invention produces the oxide/nitride/oxide (ONO) structure of flash memory devices by using semi-atmospheric pressure CVD (SAPCVD) process and tetra-ethyl-ortho-silicate (TEOS) reactants to decrease defect and interface trap and to increase reliability of the tunnel oxide layer.

[0018]FIG. 2 is a cross-section view of a flash memory device according to the present invention. The flash memory device of the present invention includes a substrate 100 and a source region 120, a drain region 140 and a channel region 160 located in the substrate 100, and a stacked gate structure 180 located on the substrate 100. The stacked gate structure 180 further includes a tunnel oxide layer 200, a floating gate 220, an interpoly dielectric layer 240, and a control gate 260. For example, the floating gate 220 and the control gate 260 are, composed of polysilicon, and the interpoly dielectric layer 240 is composed of multi-insulated layers, namely, oxide/nitride/oxide (ONO) structure. The interpoly dielectric layer 240 (ONO structure) is a stacked structure composed of a bottom oxide layer 280, a nitride layer 300, and a top oxide layer 320.

[0019] One characteristic of the present invention is the use of SAPCVD process to form the top oxide layer 320.

[0020] The reaction temperature of SAPCVD process of the present invention is, for example, between about 600° C. and about 750° C., and the preferred reaction temperature is about 680° C. The reaction pressure is, for example, between about 340 Torr and about 550 Torr, and the preferred pressure is about 400 Torr. Under such process circumstance, reactive gases, for example, TEOS and oxygen, are introduced, and following reaction (1) is also performed to form the top oxide layer 320, wherein the deposition thickness of the top oxide layer 320 is between about 20 Å and about 80Å, and the preferred deposition thickness is about 40 Å.

Si(OC₂H₅)₄→SiO₂+by-product   (1)

[0021] (Wherein the by-product is a complex mixture of organic and organosilicon compounds)

[0022] According to the above process, the reaction temperature of the present invention is lower than the reaction temperature (950° C.) of the conventional method to avoid decomposition of interpoly dielectric layer under high temperature. Another advantage of the present invention is that before reaction, the pressure of the chamber is decompressed to 500 mTorr instead of 5 mTorr disclosed by conventional LPCVD method. Moreover, gases are transported into the chamber in a continuous way to achieve vapor deposition and the waste gases are exhausted through exhausting system. Therefore, it is not necessary to vacuum the chamber to perform the next stage of the process, and the time and gas consumption problems are also dissolved.

[0023] Furthermore, the pressure of the present invention is semi-atmospheric pressure. The collision frequency of molecule is enhanced and the reaction time is shortened compared with the low-pressure circumstance of LPCVD. Moreover, in the present invention, the way of delivering the gas continuity is used to lower the turbulent flow do the phenomenon of reactant to be particle decreases. Therefore there are the preferred uniformity and quality in the top oxide layer of the deposition and lowering the thermal budget.

[0024] According to the above advantages, the present invention can improve the thermal budget and decrease charge trap to get the tunnel oxide layer with high reliability. The present invention is easier than the conventional step of oxidization to control the thickness of interpoly dielectric layer to avoid the defect caused by the too thin or too thick thickness of oxide layer in the interpoly dielectric layer and the current leakage problem caused by the defect is avoided also. Otherwise, the present invention can shorten reaction time and resolve dust problem so that an interpoly dielectric layer with high deposition quality is formed.

[0025] As is understood by a person skilled in the art, the foregoing preferred embodiments are illustrated of the present invention rather than limiting of the present invention. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure. 

What is claimed is:
 1. A structure of a flash memory, which can improve the reliability of the flash memory, the structure of a flash memory comprises: a tunnel oxide layer located on a substrate; a floating gate located on the tunnel oxide layer; and an interpoly dielectric layer located on the floating gate, wherein the interpoly dielectric layer comprises an oxide layer, a nitride layer and a semi-atmospheric pressure chemical vapor deposition (SAPCVD) oxide layer, wherein the SAPCVD oxide layer is formed by a semi-atmospheric pressure chemical vapor deposition process.
 2. The structure according to claim 1, wherein the oxide layer, the nitride layer and the SAPCVD oxide layer in a stacked structure from bottom to top are located on the floating gate.
 3. The structure according to claim 1, wherein a reaction temperature of the semi-atmospheric pressure chemical vapor deposition process is between about 600° C. and 750° C.
 4. The structure according to claim 1, wherein a reaction temperature of the semi-atmospheric pressure chemical vapor deposition process is about 680° C.
 5. The structure according to claim 1, wherein a reaction pressure of the semi-atmospheric pressure chemical vapor deposition process is between about 340 Torr and 550 Torr.
 6. The structure according to claim 1, wherein a reaction pressure of the semi-atmospheric pressure chemical vapor deposition process is about 400 Torr.
 7. The structure according to claim 1, wherein the semi-atmospheric pressure chemical vapor deposition process further comprises the use of tetra-ethyl-ortho-silicate (TEOS) and oxygen.
 8. The structure according to claim 1, wherein a thickness of the SAPCVD oxide layer is between about 20 Å and 80 Å.
 9. The structure according to claim 1, wherein a thickness of the SAPCVD oxide layer is about 40 Å. 